1. Field of the Invention
The invention relates to thick film varistors and somewhat more particularly to glass-free thick film varistors and a method of producing the same wherein varistor materials having ZnO as a main component thereof are admixed with an organic binder to form a varistor paste, which is applied onto an insulating substrate and then converted into a desired thick film varistor by sintering such paste.
2. Prior Art
Varistors are voltage-dependent impedence devices which must exhibit the highest possible impedence up to a specific voltage, the so-called varistor threshold voltage. In such devices, when the voltage is increased past the varistor threshold voltage, a steep conductivity rise occurs. The current-voltage characteristics of a varistor can be expressed by the following equation: EQU I=(V/C).sup.n
wherein I is the current flowing through the varistor, V is the voltage applied across the varistor and the exponent n is a numerical value characterizing the so-called "steepness" of the varistor. The numerical value of such steepness, n, should be as high as possible, as this steepness determines the degree to which the varistor departs from general ohmic characteristics.
Known varistors are normally produced as discrete components, typically by pressing and sintering pulverized varistor materials which have various main components, such as silicon carbide, silicon dioxide, selenium, etc. U.S. Pat. No. 3,725,836 suggests thick film varistors having a main component of ZnO and producing such varistors via thick layer techniques and thereby directly integrate such varistors into thick layer integrated circuits. In order to produce such known thick film varistors, which, as indicated above, belong to the family of ZnO-varistors, the varistor materials are mixed with glass frit and an organic binding agent to form a screen-printable varistor paste and applied via screen printing techniques onto an insulating substrate, which is then subjected to sintering conditions in order to form the desired varistors. Electrodes required for contacting such varistor can also be mounted or applied on the surface of the varistor via thick layer techniques. The steepness, n, of thick film varistors produced in this manner has a magnitude ranging between 4 and 8, which is too low for most applications.